1. Field of the Invention
The present invention relates to rotating media storage devices. More particularly, the present invention relates to an apparatus and method for repeatable runout (RRO) determination within a rotating media storage device.
2. Description of the Prior Art and Related Information
A conventional disk drive has a head disk assembly (“HDA”) including at least one magnetic disk (“disk”), a spindle motor for rapidly rotating the disk, and a head stack assembly (“HSA”) that includes a head gimbal assembly (HGA) with a transducer head for reading and writing data. The HSA forms part of a servo control system that positions the transducer head over a particular track on the disk to read or write information from that track.
Each surface of each disk conventionally contains a plurality of concentric data tracks angularly divided into a plurality of data sectors. In addition, special servo information is provided on each disk or on another disk to determine the position of the head. The most popular form of servo is called “embedded servo” wherein the servo information is written in a plurality of servo wedges that are angularly spaced from one another and interspersed between data sectors around each track of each disk. Each servo wedge generally comprises a track identification (ID) field and a group of servo bursts (an alternating pattern of magnetic transitions) which the servo control system samples to align the transducer head with or relative to a particular servo track or one or more corresponding data tracks. The servo control system moves the transducer head toward a desired track during a coarse “seek” mode using the track ID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a fine “track follow” mode. The transducer head generally reads the servo bursts to produce a position error signal (PES) that is 0 when the transducer head is at a particular radial position.
The general goal of the servo control system is to control the transducer head position relative to a desired position—i.e. to get it there and to keep it there. There are numerous outside influences, which make it difficult for the servo control system to achieve the desired transducer head position, but a particularly troublesome influence is known as “runout.”
Runout generally refers to deviation from perfect circular motion and, more particularly, refers to variation in the distance between an external point of reference and a passing surface of a rotating object. “Repeatable runout” involves periodic deviations that occur with predictable regularity (hereinafter “RRO”). “Nonrepeatable runout” involves random perturbations due, for example, to spindle runout due to bearing imperfections, and so on (hereinafter NRRO).
In the context of a disk drive, RRO is “repeatable” because it occurs in synchronization with the spinning disk. RRO comes from one or more of the following sources:    a) spindle motor runout;    b) disk slippage;    c) disk warping; and    d) disturbances converted to RRO during the Servo Writing process due, for example, to NRRO, vibrations, resonances, media defects, or disk distortion due to clamping of the HDA.
RRO may also be caused by electromagnetic imperfections due to low quality servo bursts, even if they were mechanically recorded on the ideal circle. This is true because the low quality bursts will yield incorrect position information.
Is Determining the RRO of a disk for use in track following is important because excessive RRO may cause track mis-registration (TMR) problems. Methods of determining RRO and then canceling it by one of a variety of feed-forward techniques are already well known in the art. For example, today, a common method of determining RRO is a time averaging method, in which each servo wedge value is averaged a multitude of times (e.g. hundreds of times). Consequently, the time averaging method requires a multitude of disk rotations to determine the RRO. Thus, present methods of determining RRO require the disk of the disk drive to rotate several hundreds of times before the RRO can be accurately determined.